With the recent trend toward large-sized displays, there is an increasing demand for flat panel displays, such as liquid crystal displays (LCDs) and plasma display panels (PDPs). These flat panel displays have slow response time and limited viewing angle compared to CRTs. Under such circumstances, other types of displays, typified by electroluminescence devices, are currently being investigated to replace flat panel displays.
Conventional electroluminescence devices are based on the phenomenon that inorganic semiconductors (for example, ZnS and CaS) having p-n junctions emit light when an electric field is applied thereto. However, the conventional inorganic electroluminescence devices require a driving voltage of at least 220 V AC and are fabricated under vacuum, making it difficult to fabricate in large sizes. Particularly, blue light with high efficiency is difficult to obtain from the conventional inorganic electroluminescence devices.
In attempts to solve such problems, organic electroluminescence devices (for example, organic light-emitting diodes (OLEDs)) using organic materials are being investigated. Organic electroluminescence devices use self-luminous organic materials and are based on the principle of electroluminescence in which when an electric field is applied to an organic material, electrons transported from a cathode and holes transported from an anode recombine in the organic material layer to produce energy, which is emitted as light. In comparison with LCDs, organic electroluminescence devices have the advantages of large viewing angle, low power consumption, and fast response, enabling processing of high-quality images. Due to these advantages, organic electroluminescence devices are attracting attention as next-generation display devices.
The phenomenon of light emission from organic electroluminescence devices can be largely divided into fluorescence and phosphorescence. Fluorescence refers to a phenomenon wherein light emits when an organic molecule decays from the singlet excited state back to the ground state, while phosphorescence refers to a phenomenon wherein light emits when an organic molecule decays from the triplet excited state back to the ground state.
Electrophosphorescence devices were developed by a team led by Professor S. R. Forrest at the Princeton University and Professor M. E. Thompson at the USC in 1999. The electrophosphorescence devices have markedly improved luminance efficiency compared to organic electroluminescence devices. Particularly, since spin-orbit coupling is proportional to the fourth power of atomic number, complexes of heavy atoms, such as platinum (Pt), iridium (Ir), europium (Eu), and terbium (Tb), are known to have high phosphorescence efficiency. The lowest triplet exciton of a platinum complex is a ligand-centered (LC) exciton but that of an iridium complex is a metal-ligand charge transfer (MLCT) exciton. Accordingly, the iridium complex forms stronger spin-orbit coupling and exhibits higher phosphorescence efficiency with much shorter triplet exciton lifetime than the platinum complex.
In this regard, C. Adachi et al. reported an organic electroluminescence device having a maximum luminance efficiency of ˜60 lm/w and a maximum internal quantum efficiency of ˜87% by doping bis(2-phenylpyridine)iridium(III) acetylacetonate [(ppy)2Ir(acac)], a green phosphorescent dye whose central metal is iridium, into 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole (TAZ). Further, Universal Display Corp. (UDC, USA) released that a high luminance efficiency of 82 lm/W was achieved by doping the green phosphorescent dye into a light emitting layer and using a hole injecting material developed by LG Chem. (Korea).
Organic electrophosphorescence devices capable of emitting blue, green, and red light were developed, but organic electrophosphorescence devices that can emit three primary colors and are excellent in terms of luminance efficiency, color coordinates, and lifetime, have not been reported, to our knowledge.
In this connection, U.S. Pat. No. 7,250,512 discloses iridium (III) bis(2-(2-benzothienyl)pyridinato-N,C2)(acetylacetonate) [Ir(btp)2(acac)] as a red-emitting iridium complex. However, there is still a need for a novel compound that is satisfactory in terms of color purity, efficiency, and solubility.